In the LTE (Long Term Evolution), which is next generation communication technology, a downlink uses the OFDMA (Orthogonal Frequency-Division Multiple Access) and an uplink uses SC-FDMA (Single-Carrier Frequency Division Multiple Access) for the purpose of the transmission of high-rate data. The LTE provides the 3GPP (Third Generation Partnership Project) with competitive wireless access technology.
A wireless communication system should provide terminals that have mobility with continuous service, in contrast with a wired communication system. That is, when a User Equipment (UE) moves from one region to another region, the wireless communication system has to enable the UE to continuously use the service.
Specifically, when the UE moves closer to another enhanced Node B (eNB) than an eNB that is currently providing the UE with the service, the UE has to be able to receive the service from the closer eNB. Therefore, in the wireless communication system, a handover operation, in which the UE terminates communication with a serving eNB from which the UE currently receives the service and initiates communication with a new eNB of a target eNB, has to be performed.
Additionally, in a wireless communication system, the handover operation is performed based on a system parameter for each cell according to a movement speed of the UE which is classified as one of a high speed (120 km/h or higher), a middle speed (between 30 km/h and 120 km/h), and a low speed (between 1 km/h and 30 km/h). Therefore, when the UEs move in a different movement speed equal to or higher than 120 km/h in the conventional wireless communication system, all of the UEs cannot help performing the handover based on the identical system parameter for each cell. In this respect, in the conventional wireless communication system, the UEs have no choice but to use handover control information predetermined for each cell in the performance of the handover. Further, the conventional handover has a problem in that the time for measuring RSRP (Reference Signal Received Power) and transmitting a measurement result to the serving eNB by the UE is longer than a movement time of the UE. Therefore, the high-speed UE cannot perform the fast handover, such that it inevitably has a RLF (Radio Link Failure) problem of connection interruption of a wireless link during the performance of the handover.
Hereinafter, a problem according to a movement speed of a UE in a wireless communication system during the performance of the handover will be described with reference to FIG. 1.
FIG. 1 is a graph illustrating a movement distance of a low-speed UE and a high-speed UE in the conventional wireless communication system.
Referring to FIG. 1, a movement distance 100 of the low-speed UE which moves at a speed of 120 km/h or less during a triggering time of the handover is shorter than a movement distance 120 of the high-speed UE which moves at a speed faster than 120 km/h. Therefore, a location 140 at a handover performance time of the low-speed UE is inevitably different from a location 160 at a handover performance time of the high-speed UE.
When the UE moves at a low rate, there may occur a ping-pong phenomenon due to the effect of shadowing, fast fading, and such. Further, there a problem may occur in that the handover is first performed before the UE moves to a target cell, thereby resulting in the RLF.
In addition, the high-speed UE is closer to a target eNB than a serving eNB at the time of transmitting a DL reference signal strength, i.e. DL RSRP, to the serving eNB. Therefore, an SINR (Signal to Interference plus Noise Ratio) from the serving eNB decreases and a generation possibility of the RLF increases in a corresponding location of the high-speed UE. Further, a handover delay problem may occur in that even when the high-speed UE has moved to a location within the target cell, the handover is not performed, such that the RLF may be incurred due to the delay of the handover.
Furthermore, in the event that the high-speed UE suddenly changes a route during the preparation of the handover to the target eNB after the deciding to handover from the serving eNB to the target eNB based on an RSRP measurement result reported from the high-speed UE, there is a problem of failure of the prevention of the RLF generation.
In general, in the LTE communication system, the UE measures the DL RSRP of multiple eNBs in a long period, e.g. in a period of 200 ms. Furthermore, in the event of the generation of an event related to the handover in accordance with an event-triggering scheme, the UE reports the measured RSRP to the serving eNB.
However, the aforementioned method is the method used for the low-speed UE, such that when the aforementioned method is used in the high-speed UE, many problems according to the performance of the handover are created. That is, when the high-speed UE measures the DL RSRP of the multiple eNBs in a long period similar to the low-speed UE, a distance difference between a location at the measurement time of the DL RSRP and a location at the report time of the measurement result of the DL RSRP to the serving eNB is large in comparison with the low-speed UE, such that the accuracy and the efficiency of the handover are deteriorated.
Therefore, there is a request for a method for performing efficient handover without service interruption due to the RLF even when the UE moves at a high rate.